This invention relates to devices for measuring and testing wet cell storage batteries to measure the total charge level as well as the charge level of individual cells, and especially to a device with a voltmeter having circuitry for dual range readout and with conductive probes adapted to be immersed in the electrolyte of the individual battery cells. More particularly the invention relates to a unique and particularly advantageous construction for the test probes to be immersed in the electrolyte, whereby the electrical contact between the probes and the electrolyte provides a steady, uniform, reliable flow of current during testing over an extended span of use.
Where conductive leads for wet cell storage battery testing instruments are used to measure the charge level of individual cells, the probes at the ends of the leads must be dipped in the electrolyte. This presents special and unique problems due to the chemical effect of the acid solution on most electrically conductive metals. For example, the acid solution corrodes many metals and also causes in some instances polarization after a short period of use with resulting unreliable meter readings. Also the probes must be fabricated of a metal which can be readily connected with sound electrical contact to the flexible test leads which are usually stranded wire conductors.
Some conductive metals such as ferrous metals corrode quickly due to acid action. When steel probes are immersed in the electrolyte the acid eats through the first layer of impurities on the surface of the metal in several seconds and during that time the variation in current flow prevents accurate readings. Steel probes, however, do not polarize and for that reason are effective after the initial chemical action subsides. Over a period of time, however, steel probes rust and become unsuitable, also, steel probes after use must be placed in a neutralizing solution such as baking soda to stop the corrosive effect of the acid.
Probes formed of pure nickel also tend to give a variable readout during the first several seconds of their immersion in the electrolyte and additionally they become polarized after about ten seconds.
Pure cadmium test probes also give an inaccurate variable reading for the first several minutes of their immersion or longer and when the acid action stabilizes they provide a reading voltage unpredictably lower than the actual voltage probably due to polarization. Also, the cost of pure cadmium is prohibitively high for most applications.
Cadmium plated steel probes are suitable in many respects but have a slight drift due to polarization in inaccurate readings due to a progressive drop in current flow. Accordingly, satisfactory conductive probes for storage battery testing instruments have not been obtainable in the prior art since known conductive metals and combinations thereof have inherent deficiencies that have prevented optimum utilization of the instrument's capabilities.
The present invention, however, reduces the difficulties and deficiencies indicated above and affords other features and advantages heretofor not obtainable.